On August 12, 1981 IBM introduced the IBM 5150, their first PC model. It ran an initial version of DOS acquired from a small company named Microsoft. And the world has never been the same. See
http://www.pcworld.com/article/3106730/hardware/inside-the-ibm-pc-5150-the-first-ever-ibm-pc.html for a nice description. Happy Birthday, PC!